1. Field of the Invention
The Invention concerns a toughening agent composition for organosilicate glass dielectric films. More particularly, the invention pertains to a method for restoring hydrophobicity to the surfaces of organosilicate glass dielectric films which have been subjected to an etching or ashing treatment in such a way as to remove at least a portion of previously existing carbon containing moieties, resulting in a film having reduced hydrophobicity. These toughened films are used as insulating materials in the manufacture of semiconductor devices such as integrated circuits (“ICs”), in order to ensure low dielectric constant and stable dielectric properties in these films.
2. Description of the Related Art
As semiconductor devices scale to lower technology nodes, the requirement for lower and lower dielectric constant (k) has been identified to mitigate RC delay. Similarly, as feature sizes in integrated circuits are reduced, problems with power consumption and signal cross-talk have become increasingly difficult to resolve. To achieve lower k (2.6-3.0) in dense inorganic materials, carbon has been added to reduce the polarizability thus reducing k. To achieve ultra low k (<2.4) materials, porosity is added to the carbon-rich dense matrix. While the introduction of carbon and porosity have reduced k, new challenges during back end of the line processing have also been identified. Specifically during etching and ashing, reactive gases have been found to damage the carbon at the surface of dense materials. Porous low k's have even more catastrophic effects from reactive etch and ash gases due to diffusion through the film, which causes a greater extent of damage at the internal pore walls. Once the carbon has been damaged, the films rehydroxylate and hydrogen bond with water. Because water has a dielectric constant of 70, small amounts that are absorbed for dense materials and adsorbed for porous materials cause the dielectric constant to go up significantly. Also, porous materials tend to void after copper annealing due to the high tensile stress fields which will destroy device yields. None of these are acceptable and lead to unviable materials.
It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems. While there have been previous efforts to apply low dielectric constant materials to integrated circuits, there remains a longstanding need in the art for further improvements in processing methods and in the optimization of both the dielectric and mechanical properties of such materials. Device scaling in future integrated circuits clearly requires the use of low dielectric constant materials as a part of the interconnect structure. Most candidates for low dielectric constant materials for use in sub-100 nm generation ICs are carbon containing SiO2 films formed by either CVD or spin-on methods. During subsequent processing steps, such as plasma etching and photoresist removal using plasma or wet strip methods, significant damage occurs to these low-k materials, that causes fluorine addition and carbon depletion from the low-k material adjacent to the etched surface. In addition to a higher effective k, the resultant structures are susceptible to void formation, outgassing and blister formation. The voids in turn may cause an increase in leakage current at elevated voltages and reduction in breakdown voltage. The present invention describes a way to reduce the damage and resulting issues by treating the wafers with silylating agents after the damage is caused. The use of non-damaging ash chemistry, such as H2/He has been reported to reduce carbon depletion and associated problems. In this regard, see I. Berry, A. Shiota, Q. Han, C. Waldfried, M. Sekiguchi, and O. Escorcia, Proceedings—Electrochemical Society, 22, 202 (2002); and A. Matsushita, N. Ohashi, K. Inukai, H. J. Shin, S. Sone, K. Sudou, K. Misawa, I. Matsumoto, and N. Kobayashi, Proceedings of IEEE International Interconnect Technology Conference, 2003, 147 (2003). Alternatively, post-ash treatments that replenish carbon have also been shown to restore hydrophobicity and lower the dielectric constant. Post-ashing treatments that replenish carbon have also been shown to restore hydrophobicity and lower dielectric constant. In this regard, see Y. S. Mor, T. C. Chang, P. T. Liu, T. M. Tsai, C. W. Chen, S. T. Yan, C. J. Chu, W. F. Wu, F. M. Pan, W. Lur; and S. M. Sze, Journal of Vacuum Science & Technology, B, 2 (4), 1334 (2002); and P. G. Clark, B. D. Schwab, and J. W. Butterbaugh, Semiconductor International, 26 (9), 46 (2003). An advantage of the later approach is that it allows the use of well-established etching and ashing processes. To this end, it would be desirable to repair damage caused to a porous SiCOH-based low-k material using a post-ash treatment.
One way to approach this challenge is to repair the damaged area on dense surfaces, or in the case of porous materials on the internal pore walls with a re-methylating compound called a toughening agent (TA). Toughening agents react with the damaged re-hydroxylated surfaces and re-alkylate or re-arylate them which in-turn restores the dielectric constant. The following reaction describes the an exemplary-re-methylation process: SiOH (damaged surface)+RxSi(OCOCH3)y (TA) yields SiOSiRx (repaired surface)+(CH3COOH)y (acetic acid). In the case of porous damaged internal pore wall surfaces, the re-methylation prevents void formation. Many times, the use of a toughening agent allows for conventional etch and ash processes to be utilized with low and ultra low dielectric constant materials. The treatment could result in replenishment of carbon to the low-k film, thereby restoring hydrophobicity and resistance to further damage during a wet cleaning operation. Additionally, it would be desirable if the repaired low-k material was found to be resistant to void formation, which generally occurs in untreated porous low-k inter-level dielectric regions during copper annealing processes. Silylating agents (“toughening agents”) can methylate the surface of SiO2 based materials. Contemplated exposure includes vapor exposure (with or without plasma), spin coating and supercritical CO2. Normally, SiCOH based porous low-k materials are susceptible to void formation in ILD during Cu damascene processing. After a toughening agent treatment, the resulting structure is significantly more resistant to void formation. Without being bound to any specific theory or mechanism, it is believed that plasma damage causes carbon depletion in the dielectric, by replacing Si—CH3 bonds with Si—OH bonds. In damaged porous dielectrics, the pore surface is now covered with Si—OH bonds. In the presence of tensile stress (such as after Cu annealing), adjacent Si—OH groups can condense, thus causing local densification. The evolving reaction products and the stretching of the molecules due to the new links formed, causes voids to occur near the center of the ILD space. Toughening agents prevent void formation by replacing most Si—OH bonds by Si—O—Si—Rx bonds, which avoid condensation reactions. Therefore void formation does not occur.
In addition, it is also known that existence of the SiO—SiR2—OSi linkage (where the SiR2 is one example of a toughening functionality within the matrix), that the modulus of the porous material should improve. Modulus retention and improvement is required for most porous materials to withstand imposed stresses. The toughening linkage studied, a dimethylsilyl linkage, clearly improves the modulus. If applied to weakened areas of the silicate, an improvement of the material to external stress is expected.
The toughening treatment performed after dielectric trench and via formation and etching and ashing steps repairs carbon depletion and damage to the low-k materials. By this means, voids are deterred and the later can withstand internal stresses caused by annealing treatments to the metal filling the trenches and vias.
The toughening treatment is conducted by exposing the wafer surface to the silylating agent in liquid or gas form for a period sufficient to complete the reaction with the damaged low-K region. Optionally, a high temperature bake can be performed to remove remaining solvent and excess toughening agent. Also, optionally, a wet cleaning operation can be performed immediately after the toughening agent application, or after the baking step, using a commercially available chemical compatible with the low-k dielectric. Additionally a dehydration bake may be performed before the toughening agent treatment, to increase effectiveness of the toughening agent treatment.
The effectiveness of the toughening agent treatment can be verified using unpatterned low-k dielectric films subjected to etching and ashing processing followed by the toughening agent treatment. A successful toughening agent treatment results in increased carbon concentration that can be measured by FTIR, EDX, or XPS techniques. Additionally, a water contact angle increase is seen, which demonstrates the hydrophobic nature of the post-treatment surface. The toughening agent treated film also shows a lower dielectric constant extracted from C—V measurements, compared to an etched/ashed film that is not treated with toughening agent. In patterned wafers, the effectiveness of the toughening agent treatment is demonstrated by reduction or elimination of voids in the low-k dielectric in narrow spaces between Cu trenches after a copper anneal treatment following electroplating of copper, and also by lower profile change in trenches or vias after exposure to reactive solvents.
It has been found that toughening agents are made by using silane based monomers with reactive leaving groups together with an activating agent selected from the group consisting of an amine, an onium compound and an alkali metal hydroxide.